65mm Heinrici hot air Engine

65mm Heinrici hot air Engine

Type: Closed (Stirling) cycle unpressurised hot air engine.
Piston dia.: 65mm.  Stroke: 65mm.  Swept volume: 200cc.  Flywheels diameter: 259mm.
Layout: Inverted vertical, concentric piston and displacer (beta configuration).
Power: Fractional; a few watts at 400rpm.
History: Collected in 2012 from the Temuka area, probably made in the 1920’s.

Hot air engines are external combustion engines and can be closed or open cycle, valved or valveless.

Heinrici hot air engines are of the valveless, closed cycle type, generally called Stirling cycle engines, after Robert Stirling, the Scottish Presbyterian minister who pioneered their development in the early 1800’s. They operate by alternate heating and cooling of a quantity of air, called the working fluid, contained in the engine’s internal spaces.  Heat is applied externally and passes through the cylinder wall, heating the working fluid, which is then expanded against a piston to do mechanical work.  After heating and expanding, the working fluid is moved to a cool space where it cools and contracts before being returned to the hot space for the cycle to repeat. 

It has a displacer (just a loose-fitting piston), below and in the same cylinder as the power piston to which it is connected via cranks and linkages so as to lead by 90degrees of crankshaft angle.  The displacer space and the piston space are connected by the annular gap around the displacer so that the air working fluid moves between these spaces, changing volume, pressure and temperature as the engine rotates.  Because they have no valves and experience no sudden pressure changes, Stirling engines are noted for quietness and reliability.

The other main type of hot air engine, (generally called an Ericsson cycle engine), uses valves to control the movement of air internally and can be either closed or open cycle.  Open cycle hot air engines admit a fresh quantity of air into the heating space every stroke. Confusingly, Ericsson also made valveless closed cycle engines- Stirling engines- most commonly the Ericsson pumping engine style of which there is an example in this collection.

Heinricis use air at atmospheric pressure for their working fluid, but for higher specific output (power for size) and better efficiency, modern Stirling cycle engines use pressurised gas- air, nitrogen, helium or hydrogen.

Most Heinricis in New Zealand were imported by a firm of Christchurch glaziers (Bradley Bro’s Ltd) in the late 19th and early 20th centuries.  They re-badged them to the Bradley name.  Made in a range of sizes, Heinricis were a very successful design and made to a high standard of engineering for the time.  Their most popular engine was this model – 65mm bore with 259mm flywheels.  Only suitable for low power applications, Heinricis powered water pumps, gas pumps, jeweler’s lathes, dentist’s drills, and even shop window displays. 

This example has had many hundreds of hours use and runs very well.  It is in original condition and has not been extensively restored.

Peter Lynn for the Mahan Heritage Centre 2023